Like all places, they vary. My DD used to go to a preschool, but when we moved, we put her in to a nursery (part time) for children aged 2 to 5. And, wow the difference! The nursery is ten million times better than the preschool we previously used, and I would never go back. Neither would my daughter, who absolutely loves her new nursery.
At my daughter's nursery, almost all the staff are married and mothers. Only 1 /2 of the workers are younger. About 3/4 staff are in their 30s or older. The opportunities they provide for the children have been amazing. Last term, the theme was pets and they arranged to have visits from a tortoise, hamster, cat, dogs, rabbits even a pony! They all got to groom the pony and my daughter loved it. At her nursery, they have hens that they collect eggs from, guinea pigs that they look after, a large outside play area and it is all so calm and quiet. The children are just so happy there. I've never seen any child cry in the mornings and not want to be dropped off. In fact, I once had tears that my daughter cried when we were leaving, as she didn't want to go, and you could tell by the children and staffs reaction, that it was a nursery where they're not used to tears! (Apart from bumps and bruises).
The only issue I have is on a friday, she is cared for by her grandfather. We regularly have tears because she doesn't want to go, as she wants to go to nursery instead. Over half term, we had tears because she wanted to go to nursery and not have the week off.
I now, would never go back to using a preschool. The nursery we are now using really made us reevaluate the preschool we previously used. Sure, DD was happy enough there, but she didn't love it like she loves nursery.
So, this is just my experience of using one nursery... but it would be ridiculous of me to make a blanket statement like "Parents shouldn't use pre schools, they're inferior to paid nurseries, and so they should send them there instead". Of course, there are fantastic pre schools around too, just like there are fantastic childminders. And of course, there are rubbish nurseries, pre schools, childminders and parents as well. We can't generalise, all we can do is to do the best for our children!